Restore ordinary exercise state after Run or Submit, including failures.
An unsuccessful cleanup makes the kernel ineligible for reuse. This is
an ordinary-state guarantee, not isolation from arbitrary hostile code.
Installed after configured startup, once per kernel incarnation.
Returns string
prepare
prepare(_spec): string
Restore the ordinary-state baseline and prepare the attempt. Policies may
retain authored setup in run() to preserve existing error/output behavior.
Partial setup failure must still permit cleanup.
Native Python keeps its subprocess grader and shares ordinary attempt cleanup.